Measurements of elemental composition were obtained from synaptic structures in the molecular layer of mouse cerebellar cortex in order to determine physiological changes that occur after neuronal activity. X-ray microanalysis was performed on cryosections freeze-dried in the field emission STEM. Data were recorded mainly from post-synaptic endoplasmic reticulum cisterns situated in Purkinje cell dendrites and in dendritic spines, but analysis was also performed on pre-synaptic terminals, neuroglia, mitochondria, cytoplasm, and other structures.